Thermocolor ribbons usually comprise a foil-like carrier which may, for instance, consist of paper or plastic, and a layer of a melt-applied color, particularly in the form of a layer of coloring agent or carbon black bound to wax. In these thermocolor ribbons, the melt-applied color is melted by means of a thermal printing head, and transferred to a recording paper. In this instance, one speaks generally of a TCR ribbon ("Thermal Carbon Ribbon"). Thermal printers which impress a thermal character during the printing process are known, e.g. from DE-OS 2 062 494 and 2 406 613 as well as from DE-OS 3 224 445. During the printing process, the procedure is, in detail, as follows: the print head of a thermal printer presses the thermocolor ribbon against the recording paper, thereby causing the print head to develop temperatures which may maximally lie at about 400.degree. C. The uncoated backside of the thermocolor ribbon or the foil-like carrier is, during the printing operation, in direct contact with the print head or the thermal symbol formed thereon. At the instant of the printing operation proper, the relative speed between the thermocolor ribbon and the printing paper is zero. The melt-applied color is transferred from the thermocolor ribbon to the printing paper through a melting process in the shape of the character to be imprinted. Upon disengagement of the thermocolor ribbon, the melted symbol remains attached to the printing paper and solidifies.
In addition to the thermocolor ribbons described above, with simple foil-like carriers, there are also thermocolor ribbons where the thermal symbol is created not by a thermal print head, but by the resistive heating of a specially designed foil-like carrier. The melt-applied color, which is the "operative layer" during the printing process, also contains the materials enumerated above. Within the art, the foregoing is called an "ETR" thermocolor ribbon ("Electro Thermal Ribbon"). A thermotransfer-printing system of this type is described in U.S. Pat. No. 4,309,117.
Thermocolor ribbons are already known which print several times (i.e. they are designed for multi-use). Such thermocolor ribbons are described, for instance, in EP-A 0 063 000. In accordance therewith, the melt-applied color of the thermocolor ribbon is of particulate material, which is insoluble in the solvent of the coating liquid and does not melt below 100.degree. C., and further incorporates a particulate material with a melting point between 40.degree. and 100.degree. C. The particulate material not melting below 100.degree. C. should preferably be a metal oxide, a metal, organic resin or carbon black. The layer os the melt-applied color which represents a solid mixture shall, by means of said special particulate material, receive a heterogeneous structure, which allows only a small quantity of the molten material to be transferred to be used-up during a single printing operation.
In connection with the known thermocolor ribbons, it has become evident that improvements are needed in their printouts.